Processing of nickel-base alloys for improved fatigue properties

ABSTRACT

The fatigue properties of the precipitation-hardened, nickelbase alloys, such as Inconel 718, Incoloy 901 and Waspaloy, are significantly improved by a thermomechanical processing technique involving the generation of an intermetallic pinning phase, such as a spheriodal eta phase or an overaged gamma prime phase, with subsequent recrystallization to provide a uniform microstructure having a grain size of ASTM 10-13 or finer.

[ 51 May 2,1972

United States Patent Brown et al.

S T m m mT as W U m U 3,519,503 7/1970 Mooreetal.......................148/11.5 3,420,716 1/1969 Slepitis..............................148/11.5

Primary Examiner-Richard 0. Dean Attorney-Richard N. James Brown, Glastonbury; Raymond C. Boeitner, Windsor, both of Conn.

United Aircraft Corporation, East Hartford, Conn.

[73] Assignee:

ABSTRACT roperties of the precipitation-hardened, nickel- [22] Filed: May1B,1970

[21] Appl.No.: 38,227

The fatigue p base alloys, such as lnconel 718, lncoloy 901 and Waspaloy, are significantly improved by a thermomechanical processing .143 L5 R, 48/115 F 43/123 technique involving the generation of an intermetallic pinning 22 1/10 phase, such as a spheriodal eta phase or an overaged gamma 148/115 R H 5 F 12 7 prime phase, with subsequent recrystallization to provide a uniform microstructure having a grain size of ASTM 10-1 3 or finer.

581 FieldofSearch........................

1 1 Claims, 4 Drawing Figures PPCiHTEn MM 2 m2 SHEET 1 Bi 2.

' Jaw SHEET 2 BF 2 PROCESSING OF NICKEL-BASE ALLOYS FOR IMPROVED FATIGUE PROPERTIES BACKGROUND OF THE INVENTION The present invention relates in general to the nickel-base alloys and, more particularly, to a novel fabrication process therefor to provide improved physical properties including increased fatigue resistance.

In the gas turbine engine industry where so many of the strong nickel-base alloys have great utility, experience has demonstrated that one of the critical factors which must be considered is the fatigue resistance of the alloys. For gas turbine engine disks and shafts, fatigue resistance, particularly low cycle fatigue resistance. may in fact be the limiting factor in the establishment of the useful operating lives of such components. Although the fatigue problem may in some instances be solved by the substitution of other materials or by alterations in alloy chemistry, such substitutions or alterations are usually made with considerable reluctance, usually for economic reasons although the drawbacks incident to the development of new experience factors with new alloys or old alloys in new applications are also of importance.

It is well known that the physical properties of engine components are dependent not only upon their alloy chemistries but also upon their fabrication histories. With proper processing specific alloys may be provided with specific property alterations or the alloy properties may in general be enhanced. One such improved processing technique is disclosed in the copending application of G. H. Rowe et a1. entitled Process for Improving the Fatigue Resistance of Certain Nickel-Base Alloys, application Ser. No. 775,541, filed Nov. 13, 1968 and assigned to the present assignee. Another advanced processing technique applicable to the nickel-base superalloys is that described in the copending application of W. A. Owczarski et a1. entitled Thermomechanical Strengthening of the Superalloys, application Ser. No. 864,268, filed Sept. 26, 1969 and also assigned to the present assignee.

Of particular interest in the present process are those nickel-base alloys of the type typified by lnconel 718, lncoloy 901 and Waspaloy, representative chemistries for these alloys being as follows:

SUMMARY OF THE INVENTION This invention contemplates the processing of certain precipitation hardened nickel-base alloys to provide improved fatigue resistance thereto. It is applicable to those nickel-base alloys precipitating intermetallic compounds, such as an eta (Ni cb, Ni Til or overaged gamma prime (Ni Al, Ti), which are stable above the alloy recrystallization temperature and which may be produced in the microstructure below the alloy recrystallization temperature. These are hereinafter collectively referred to as pinning phases.

In the fabrication sequence, the alloys are thermodynamically processed to provide a uniform dispersion of a fine pinning phase and subsequently recrystallized. The uniform dispersion of the pinning phase may be provided in a number of ways including: cold or warm working the alloy and subsequently heat treating the as-worked structure to precipitate a spheroidal eta or overaged gamma prime phase; warm working at a temperature sufficient to induce precipitation of the pinning phase during deformation; or establishment of a conventional needle-like eta phase and subsequent thermomechanical processing to effect a conversion to the desired pinning phase such as the line spheroidal eta.

In the more preferred processing sequence for the alloys capable of eta precipitation, the alloy is first subjected to heat treatments to minimize alloy heterogeneity and to precipitate the conventional needle-like eta; then warm worked to effect a conversion of the eta to a uniform fine dispersion of the desired spheroidal eta; and subsequently recrystallized to form a microstructure having a grain size of ASTM 10-13 or finer. Normally, the conventional aging heat treatments are thereafter applied for strengthening purposes.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a photomicrograph of conventionally processed 1nconel 718 bar stock revealing a comparatively coarse grain size (ASTM 4-5) with the absence of any significant amount of the eta phase. l50 before reduction).

FIG. 2 is a photomicrograph of an lnconel 718 pancake processed according to the present invention and illustrating a fine grain size (ASTM 12). (250Xbefore reduction).

FIG. 3 is a photomicrograph, at greater magnification, of the sample of FIG. 2 showing a uniform distribution of spherical eta particles refining grain size. 1 ,000Xbefore reduction).

FIG. 4 is a graph plotting the fatigue resistance of lncoloy 901 as a function of grain size.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The demands for improved engine performance and increases in engine operating temperatures, and product improvement programs designed to increase the operating lives of engine components have progressed to the point where materials limitations have been reached. In particular, gas turbine engine shafts and disks have been found to be limited by low cycle fatigue in many instances. In the study of the fatigue behavior of certain nickel-base alloys as a function of heat treatment, it was determined that ultra-fine grain sizes can provide vastly improved fatigue and strength properties. It was further determined that by suitable processing, the grain size can be substantially reduced by the precipitation of particular intermetallic pinning phases prior to recrystallization.

In the case of lnconel 718, grain size refinement can be achieved through precipitation of an eta phase prior to recrystallization. Eta in this alloy is an orthorhombic Ni Cb phase which is typically precipitated in this alloy in the l,600"-l ,700" F. temperature range and which is stable above the alloy recrystallization temperature. Normally, as eta is allowed to precipitate in fully annealed alloy, it nucleates at grain boundaries and grows preferentially along (111) crystallographic planes, forming long straight needles traversing each grain. Inasmuch as in this form it does not contribute significantly to the strength of the alloy and in fact competes for the elements forming the hardening gamma prime precipitate, most of the literature has concluded that the eta phase should be avoided.

If, however, the eta precipitate is forced to precipitate in a material which has been deformed below the alloy recrystallization temperature or otherwise properly processed, it may be provided in a uniform dispersion throughout the matrix, appearing metallographically as generally spheroidal particles 1-3 microns in size. This may be seen in FIG. 3. If the alloy is then recrystallized with the uniform dispersion of fine spheroidal eta present, the newly formed grain boundaries incorporate the eta, effectively inhibiting grain growth. The result is a much finer, more uniform grain size than that achieved by conventional processing, which may readily be observed by a comparison of the microstructures of FIGS. 1 and 2. Early indications revealed that with proper processing the grain size could be reduced from ASTM 3 to at least about ASTM 10 with an increase in fatigue resistance of about 40 percent. It is therefore now possible to provide nickel-base alloys having fatigue strengths equal to or exceeding those attainable in the iron-base alloy systems wherein the fatigue properties are often independent of grain size.

The significant property improvements have been obtained only in those nickel-base alloys displaying the fine grain size The present processing has also been applied to Waspaioy wherein the grain growth retardation function during recrystallization is provided by an overaged gamma prime precipitate. This pinning precipitates, having an average which, in turn, is a function not only of the heat treatments but 5 diameter of 0.5-] micron, are introduced into the material of the entire processing parameters and sequencing. For prior to forging or other deformation by a heat treatment of reproducibility and the attainment of optimum results, strin- 3 for 24-48 houngut gongom h gntjn f b i i Prom i mandatory, Recrystallization is a process whereby cold-worked material i l di lo e omm] f th forging or other def ation reverts to a strain-free structure by the nucleation and growth variables. IQ of new grains. In the precipitation; hardened, nickel-base al- Experience has demonstrated that a number of criteria must loys recrystallization is conducted above the solvus tempera be satisfied for effective results. The pinning precipitate must t re of the hardening phase or phases. In the lnconel 718 and be stable at temperatures in excess of the alloy recrystallizalncoloy 90! alloys, the pinning eta precipitate and the hardention temperature for the grain refining function of the "'8 8 W P characterized by divcrlem OM18 i ing, a, f on presence d k the temperatures and recrystallization is conducted at a temperarecrystallisation process. The pinning precipitate must ture above the gamma prime solvus but below that of the precipitate profusely at temperatures below the recrystalliza- P l P can of wl'paioy as conventionally temperature d i n b digpefnd nif ly mulated, the pinning precipitate and the hardening phase are throughout the microstructure. And recrystallisation must 20 essentially identical chemically and metallurgicslly, the prints occur subsequent to the formation and dispersion of the ry difference residing in the size of the precipitate. However, pinning precipitate. For practical reasons in production runs, because of this size difference, it u possible to solution the a reasonable spread between the recrystallisation temperature hardening phase while retaining the effect of the overaged and the solvus temperature of the pinning precipitate is very precipitate for grain size control, since the smaller particles much preferredashereinal'ter discussed in greater detail. are more readily solutioned. However, the recrystallization The lncoloy 90] alloy exhibits precipitation of a hexagonal process is time-limited in this alloy. Ni,'li eta phase with characteristics similar to the Ni,Cb phase Table II compares the precipitation characteristics of the observed in lnconel 718. Retardation of grain growth by a eta andthe gamma prime hardening phases in the lnconel 718 uniform dispersion of spheroidal eta has also been accomand lncoloy 90l alloys.

TABLE II [Comparison of the Pinning and Hardening Precipitation licactlons] Precipitation Maximum Chemical Crystallographic Approximate temperature stability Strengthening Phase composition structure particle size range typical tcmpcraturc characteristics Em --{i$i li12l iiiiiii liZQLQQiKHT'fFJI:JiLIl l l; #2??? 13.53} We Nl Cb (H8) Face centered Gamma primc {Nla'll (901) Cubic or body ccntcrml .}(l-500A.-K l, 000-l, 500 F. l, MP-1,800"? Major strengthening I: pf.

I Visible optically. 2 Not visible opt cally.

plished with this alloy. As compared to conventionally processed lncoloy 90] the fine grain structures processed according to the present invention exhibiting grainsizes of ASTM l0 0.0002 inch diameter) or finer display superior fatigue and tensile strength properties. Tensile and yield strengths are increased by a factor of about 10-20 percent, and the smooth high cycle fatigue (HCF) life is increased by a factor of 40 percent.

In its chemistry, lncoloy 90l does present somewhat of a practical processing problem. Whereas lnconel 7 I 8 exhibits a 50-75 F. temperature differential between the eta solvus and the recrystallization temperature, the differential for lncoloy 901 is only about l5'-25 F. Due to this very narrow differential, processing difficulties are increased with these alloys. it is, of course, possible to resolve this problem by certain modifications in alloy composition. Eta solvus temperatures were detennined for several such modifications as detailed in Table I.

Control composition.

Increasing chromium, molybdenum and titanium contents result in higher eta solvus temperatures. and for all compositions the recrystallization temperature appears to be below 1,775 F. These modified alloys can be forged to the desired fine grain condition at higher forging temperatures and over a wider temperature range than the control composition.

seat

As indicated in Table ll, eta precipitates in the lnconel 7 l 8 and lncoloy 901 alloys at the nominal chemistry in the l,500-l,700 F. range. If fully annealed material is exposed to heat treatment in this temperature range, eta precipitation will occur as a needle-like phase whereas the desired grain size refinement is dependent upon the presence of eta as a uniform distribution of spheroidal particles.

Spheroidal eta precipitation may be caused to occur under either of two processing conditions: l heat treatment of cold worked alloys of this type in the l,600- l ,700' F. temperature range, or (2) warm working below the recrystallization temperature )l,7$0 F.). in addition, spheroidal eta may be provided by warm working the alloys containing the needle-like eta below the alloy recrystallization temperature.

The extent of working is not critical as long as sufficient energy for recrystallization is imparted to the structure to provide spheroidal eta precipitation with these alloys. Room temperature deformation of lnconel 178 will generally exceed about a 40 percent reduction in thickness and that for lncoloy l a 50 percent reduction to ensure spheroidal eta precipita tion upon subsequent heat treatment.

In the case of Waspaloy, an overaged gamma prime precipitate is introduced into the material by a heat treatment at l,800'-l,825 F. for 24-48 hours. Because the precipitate so formed is large and incoherent compared to the hardening gamma prime that is produced during aging (l,400 F .j, the hardness of the material is relatively low. Subsequent working operations to the desired configuration (typically hammer forging, press forging or extrusion) at l,800-l,850 F. will provide the desired recrystallization energy at reductions of 30 percent.

The preferred processing conditions for the lnconel 7| 8 and Hunts 0114 lncoloy 901 alloys is as follows: (a) homogenization, and eta precipitation heat treatment at l,650-1,700' F. for 4-8 hours, (b) forging at a 50-65 percent reduction at or below the eta solvus temperature (1,825 P. and 1,750 E, respectively, for the above alloys) and (c) solution heat treatment with recrystallization 2$-50 F. below the eta solvus. The latter heat treatment for 1 hour is sufficient to induce recrystallization without substantial grain growth. Short time reheats in excess of those described are tolerable provided that the eta structure is not adversely affected and no substantial grain growth occurs.

After establishment of the grain size as above described, the alloys prior to use are subjected to the usual aging heat treatment for strengthening through precipitation of the hardening gamma prime phase. For Inconel 718, this comprises holding at 1,325 F. for 8 hours, and 1.150 F. for 8 hours. For lncoloy 901, the aging comprises heat treatment at 1,325 F. for 6 hours and 1,200 F. for 12 hours. Cooling rates are generally equivalent to air cool or faster.

With Waspaloy the preferred processing involves: (a) an initial heat treatment at l,800-i,825 F. for 24-48 hours to form the overaged gamma prime precipitate; forging to the desired configuration using a preheat temperature of 1,800-l ,850' F. and reductions of 30 percent or more; and heat treatment at 1,800-1,850 F. for 2-4 hours for recrystallization, providing a grain size of ASTM 10 or finer. A sustained temperature of 1,850 F. cannot be exceeded during any stage of the process subsequent to the formation of the overaged gamma prime pinning precipitate because of its instability above this temperature. A final stabilization heat treatment at about l,550 F. for 4 hours and aging at about 1,400 F. for 16 hours will provide a full hardness response.

The property improvements incident to the preferred processing according to this invention may be seen from FIG. 4 of the drawings and the following tables.

TABLE I11 [Tensile and high cyclc fatigue propcrtit-s of lnuunol 718 iiLllSttNik material processed to several grain sizes. All data llluoldnll ut ass" 1%} Ullhnutn Fatigue tnnsiln 0.2% yii-lli Elon rn- Fatigue Umin strength, strength, strength, tion, rntiu size K 5.1. K s.i. K s.l. percent 1" S/UIS ASTM 13... 114 103 158 22. 5 ii. iii) ASTM 10... )7 177 151 21. 5 (i. M ASTM 5.... 80 170 154 17.5 0. 47 AS'IM 3.... 55 166 144 20. (1.33

LCF data on fine grain Inconol 718 barstock material Tempcru- Cycles to Stress, turn, laiinrc (lrain size K s.i. F. (avg.)

ASTM l2 30:i:65. 850 1 200, 000

ASTM l2 4(1zbl5l) 1, 000 41, 000

Stress rupture data on line grain Inconul 71b hurstock inuturiul Rfl. Tunporu- Time to clongu Stress, turo, rupture, tion, Lirain size K 5.1. F. hours percent ASIM 90 1,200 293.1 13.1 ASTM 13 90 1,200 170.0 12.8 ASTM 75 1,300 27.4 16.) ASTM13 75 1,300 14.1 22.1

1 No failure.

TABLE IV [Comparison of LCF properties of fine grain lncoloy 901 with conventionally processed I ncoloy 901 materials] has [Comparison of LOF properties of lino grain lncoloy 901 with conventionally processed Incoloy 001 materials] (lruln Stress, tum, Cycles to Alloy sizc K 5.1. i failure ASTM 5. 30:1:77. 5 854) 11,000 35,000

Stross rupture data for line gruln lncoloy U01 harstock material Thus, by providing a fine uniform dispersion of a pinning precipitate, such as the spheroidal eta phase or the overaged gamma prime, prior to recrystallization, and effecting recrystallization in the presence of the pinning phase to control the grain size, it is possible to provide dramatic improvements in the fatigue resistance of nickel-base alloys of the type typified by lnconel 718, lncoloy 901 and Waspaloy.

Although the invention has been described in detail with reference to certain preferred embodiments and examples for the purpose of illustration and explanation, the invention in its broader aspects is not limited to the exact details described, but improvements to and departures therefrom may be made within the scope of the appended claims without departing from the principles of the invention and without sacrificing its chief advantages.

What is claimed is:

l. The method of improving the fatigue resistance of the precipitation hardenable, nickel-base alloys capable of precipitating inter-metallic compounds which are stable above the alloy recrystallization temperature which comprises:

thennomechanically processing the alloy to develop a fine intemietallic pinning precipitate uniformly dispersed throughout the alloy microstructure; and

subsequently recrystallizing the alloy in the presence of the pinning phase to provide a grain size of ASTM 10 or finer.

2. The method according to claim 1 wherein:

the intermetallic pinning precipitate is a spheroidal eta phase or an overaged gamma prime phase.

3. The method according to claim 2 wherein:

the intermetallic pinning precipitate has an average particle size of about 0.1-1 micron.

4. The method of improving the fatigue resistance of the precipitation hardenable, nickel-base alloys subject to eta phase precipitation which comprises:

thermomechanically processing the alloy to develop a fine spheroidal eta precipitate uniformly dispersed throughout the alloy microstructure', and

subsequently recrystallizing the alloy at a temperature below the solvus temperature of the eta precipitate to provide a grain size of ASTM 10 or finer.

S. The method according to claim 4 wherein:

the thermomechanical processing comprises cold working of the alloy followed by heat treatment near but below the alloy recrystallization temperature to precipitate the spheroidal eta phase.

6. The method according to claim 4 wherein:

the thermomechanical processing comprises warm working the alloy near but below the alloy recrystallization temperature to precipitate the spheroidal eta phase.

7. The method according to claim 4 wherein:

the thermomechanical processing comprises homogenization of the alloy and precipitation of a needle-like eta phase followed by warm working near but below the alloy recrystallization temperature to convert the needle-like eta phase to the spheroidal eta phase.

8. The method of improving the fatigue resistance of the lnconel 718 and lncoloy 901 alloys which comprises:

working the alloy to effect a compressive strain equivalent 7 8 to at least a 30 percent reduction in thickness and heat l,750-l ,825' F. to form afine spheroidal eta precipitate; treating the alloy at a temperature of l,500-l ,750" F. to and generate afine spheroidal precipitate uniformly dispersed recrystallizing the alloy at a temperature of l,725-l ,800 throughout the alloy microstructure; and F. for about I hour.

subsequently recrystallizing the alloy at a temperature II. The method of improving the fatigue resistance of the below the eta phase solvus to a grain size of ASTM [D or lncoloy 90l alloy which comprises:

finer. I eta phase precipitation heat treatment of the homogenized 9. The method according to claim 8 wherein: alloy at l,650l .700 F. for about 4-8 hours; g m the 15 heat to forging to a compressive strain equivalent to at least a 30 precipitate the hardening phase.

percent reduction in thickness at a temperature of "F "Mame mom-1,750" F. to form a fine spheroidal eta precipitate; lnconei 718 alloy which comprises:

and eta phase precipitation heat treatment of the homogenized alloy at 1,650,430, F for about hours; recrystallmng the alloy at a temperature of about L725 forging to a compressive strain equivalent to at least a 3O 7500 for abom 1 hour percent reduction in thickness at a temperature of 

2. The method according to claim 1 wherein: the intermetallic pinning precipitate is a spheroidal eta phase or an overaged gamma prime phase.
 3. The method according to claim 2 wherein: the intermetallic pinning precipitate has an average particle sizE of about 0.1-1 micron.
 4. The method of improving the fatigue resistance of the precipitation hardenable, nickel-base alloys subject to eta phase precipitation which comprises: thermomechanically processing the alloy to develop a fine spheroidal eta precipitate uniformly dispersed throughout the alloy microstructure; and subsequently recrystallizing the alloy at a temperature below the solvus temperature of the eta precipitate to provide a grain size of ASTM 10 or finer.
 5. The method according to claim 4 wherein: the thermomechanical processing comprises cold working of the alloy followed by heat treatment near but below the alloy recrystallization temperature to precipitate the spheroidal eta phase.
 6. The method according to claim 4 wherein: the thermomechanical processing comprises warm working the alloy near but below the alloy recrystallization temperature to precipitate the spheroidal eta phase.
 7. The method according to claim 4 wherein: the thermomechanical processing comprises homogenization of the alloy and precipitation of a needle-like eta phase followed by warm working near but below the alloy recrystallization temperature to convert the needle-like eta phase to the spheroidal eta phase.
 8. The method of improving the fatigue resistance of the Inconel 718 and Incoloy 901 alloys which comprises: working the alloy to effect a compressive strain equivalent to at least a 30 percent reduction in thickness and heat treating the alloy at a temperature of 1,500*-1,750* F. to generate a fine spheroidal precipitate uniformly dispersed throughout the alloy microstructure; and subsequently recrystallizing the alloy at a temperature below the eta phase solvus to a grain size of ASTM 10 or finer.
 9. The method according to claim 8 wherein: following recrystallization the alloy is heat treated to precipitate the hardening phase.
 10. The method of improving the fatigue resistance of the Inconel 718 alloy which comprises: eta phase precipitation heat treatment of the homogenized alloy at 1,650*-1,700* F. for about 4-8 hours; forging to a compressive strain equivalent to at least a 30 percent reduction in thickness at a temperature of 1,750*-1, 825* F. to form a fine spheroidal eta precipitate; and recrystallizing the alloy at a temperature of 1,725*-1,800* F. for about 1 hour.
 11. The method of improving the fatigue resistance of the Incoloy 901 alloy which comprises: eta phase precipitation heat treatment of the homogenized alloy at 1,650*-1,700* F. for about 4-8 hours; forging to a compressive strain equivalent to at least a 30 percent reduction in thickness at a temperature of 1,700*-1, 750* F. to form a fine spheroidal eta precipitate; and recrystallizing the alloy at a temperature of about 1,725*-1, 750* F. for about 1 hour. 